The present invention relates to airborne radars and more particularly to controllers for airborne radars. Radar is often sequenced through various scans, with each scan or set of scans dedicated to a particular one of the tasks performed by the radar. The scan sequence and the time allotted for each scan type limits the type and number of data gathering tasks the radar can support. In present airborne radars, the radar alternates between windshear and weather detection scans at low altitude. In one commercially available radar, one scan gathers weather data and three scans detect windshear. Even at these current rates, the amount of weather data gathering capability is marginal. Additional data gathering tasks cannot, therefore be added to the radar scan schedule of existing radars without compromising weather radar performance.
Present day airborne radar systems also require that the pilot manually control the tilt angle of the radar antenna in order to scan for the desired data usually weather. An example of manually setting tilt is described in "RDR-4B Forward Looking Windshear/Weather Avoidance Radar System Pilot's Manual with Radar Operating Guidelines," by AlliedSignal Aerospace Commercial Avionics Systems, ACS-5082, Rev 1, July '96, the entire contents of which are incorporated herein by reference.
The pilot manual describes the procedure for selecting the antenna tilt to scan for weather. This procedure requires the pilot to adjust manually the tilt of the antenna for each range scale until "a sprinkle of ground return" is visible at the far edge of the display. At the larger range scales (&gt;80 nm) the ground returns may not be visible making an optimal antenna tilt decision difficult due to the lack of terrain returns. At these ranges, it is difficult for the pilot to make a distinction between weather returns and strong ground clutter returns without continually adjusting the antenna tilt to see if the returns disappear as the antenna beam is adjusted upward. As the altitude of the aircraft changes with respect to target height, the antenna tilt angle must be adjusted to maintain the proper positioning of the radar beam with respect to the target. This requirement increases pilot workload, and presents possible difficulties in maximizing the effectiveness and utility of the radar system. Furthermore, pilots must also make periodic adjustments to the weather radar tilt to maintain an optimal weather viewing tilt angle.
There are two different automatic tilt capabilities on general aviation radars:
Automatic tilt based on barometric altitude and range selection. PA1 Automatic tilt angle compensation based on altitude changes.
In the first implementation, the radar receives the barometric altitude from the air-data computer and calculates a tilt angle to have the radar beam hit the ground at the selected display range. Since the automatic tilt angle calculation is based on the barometric altitude, not absolute altitude above the ground, this method can result in different levels of ground clutter in the display depending on the local pressure conditions as well as the factual terrain altitude. Pilot acceptance of this method of automatic tilt has been limited at best. It is definitely not suitable for air transport flight crews.
With the second implementation, the pilot is allowed to make an initial setting of the tilt angle. This eliminates problems associated with the first method. Then, if the automatic tilt control is activated, the system automatically compensates for the required tilt changes as the aircraft altitude changes. It is basically an automatic altitude change compensator. This method, however, unrealistically assumes that the terrain ahead of the aircraft is flat or otherwise unknown. This method is thus still subject to variations in the ground clutter when the aircraft flies over different terrain altitudes.
Accordingly, improvements to existing tilt control systems are needed in the industry.